Explain ECG

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I now have a thorough body of information from Harrison's, Goldman-Cecil, Miller's Anesthesia, and Gray's Anatomy. Let me compose the full answer.

Electrocardiogram (ECG)

What Is an ECG?

An electrocardiogram (ECG) is a non-invasive recording of the electrical activity of the heart at the body surface. It captures differences in electrical potential generated by the heart between two recording sites. Because the heart's electrical events are directly linked to mechanical contraction, the ECG provides a window into both the electrophysiology and, indirectly, the mechanical function of the heart.

Harrison's Principles of Internal Medicine, 22E, p. 1911
Goldman-Cecil Medicine, p. 422

The Cardiac Conduction System (The Source of the ECG Signal)

Before reading an ECG, you need to understand the electrical "wiring" that generates the signal:

Structure	Location	Function
SA node	Junction of SVC and right atrium (crista terminalis)	Primary pacemaker; spontaneously depolarizes at the highest rate
Bachmann bundle	Inter-atrial conducting tract	Speeds depolarization to the left atrium
AV node	Near coronary sinus opening, interventricular septum	Physiologic conduction delay (slows signal before it reaches ventricles)
Bundle of His	Lower border of membranous interventricular septum	Rapid-conduction bridge between AV node and bundle branches
Right & Left bundle branches	Right and left sides of interventricular septum	Carry impulse to respective ventricles
Left anterior/posterior fascicles	Divisions of left bundle branch	Subdivide activation of the large left ventricle
Purkinje fibers	Subendocardial network	Final distribution to ventricular cardiomyocytes; insulated from myocardium until contact points

The AV valve rings are electrically insulated, so the AV node is normally the only electrical connection between atria and ventricles - this is critical for preventing re-entrant arrhythmias.

Gray's Anatomy for Students, p. 414
Goldman-Cecil Medicine, p. 422

ECG Paper and Calibration

The ECG is recorded on graph paper divided into 1 mm² boxes:

Horizontal axis = time: at standard speed of 25 mm/s, each small box = 40 ms (0.04 s), each large box (5 small) = 200 ms (0.20 s)
Vertical axis = voltage: standard calibration = 1 mV = 10 mm

Heart rate can be estimated by dividing 300 by the number of large boxes between consecutive R waves (or 1500 by the number of small boxes).

Harrison's Principles of Internal Medicine, 22E, p. 1912

The ECG Waveforms and Intervals

Here is a diagram showing all the key waves and intervals:

Basic ECG waveforms: P wave, QRS complex, ST segment, T wave, U wave, and labeled PR, QRS, and QT intervals

And below is a diagram showing how ECG events map to the cardiac cycle (ventricular pressure, volume, heart sounds):

Cardiac cycle diagram correlating ECG waves with ventricular pressure, aortic flow, ventricular volume, heart sounds, and venous pulse over time

P Wave

Represents atrial depolarization
Normally positive in lead II, negative in aVR (because the SA node impulse travels downward and to the patient's left)
A biphasic P in V1 is normal: positive component = right atrial depolarization; small negative component = left atrial depolarization
Atrial repolarization is buried under the QRS and usually not visible

PR Interval

Measured from onset of P wave to onset of QRS
Normal: 120-200 ms (3-5 small boxes)
Represents conduction through atrial tissue, AV node, and His-Purkinje system
Prolonged PR (>200 ms): first-degree AV block (usually AV nodal delay)
Short PR: ventricular pre-excitation (Wolff-Parkinson-White), junctional rhythm, or enhanced AV nodal conduction

QRS Complex

Represents ventricular depolarization
Much larger amplitude than P wave due to the greater ventricular muscle mass
Normal duration: 75-110 ms (up to ~3 small boxes)
Naming convention: capital letters (Q, R, S) = amplitude ≥5 mm; lowercase (q, r, s) = amplitude <5 mm
- Q/q: initial negative deflection
- R/r: positive deflection
- S/s: negative deflection following a positive deflection
- QS pattern: entirely negative complex
Prolonged QRS (>110 ms) indicates intraventricular or interventricular conduction delay (e.g., bundle branch block)

ST Segment

Junction between end of QRS (J point) and beginning of T wave
Corresponds to phase 2 (plateau) of the ventricular action potential
Normally isoelectric (flat at baseline)
ST elevation: transmural (epicardial) ischemia/injury, pericarditis, Brugada syndrome
ST depression: subendocardial ischemia

T Wave

Represents ventricular repolarization (phase 3 of action potential)
Normally upright in most leads; inversion can indicate ischemia, hypertrophy, or other pathology
Deep T-wave inversions in V1-V4 (Wellens sign) = severe LAD stenosis

QT Interval

Measured from onset of QRS to end of T wave
Reflects both ventricular depolarization and repolarization
Inversely proportional to heart rate - must be corrected
Bazett's formula: QTc = QT / √RR (in seconds)
Upper normal limits: QTc ≤460 ms in women, ≤450 ms in men
Prolonged QT: hypokalemia, hypocalcemia, drugs (amiodarone, antipsychotics), channelopathies - risk of torsades de pointes
Shortened QT: hypercalcemia, digoxin, hereditary short QT syndrome

U Wave

Small deflection after T wave; normal variant, most prominent in V2-V3
Prominent U waves: hypokalemia, bradycardia
Inverted U waves: may indicate ischemia or left ventricular hypertrophy
Harrison's Principles of Internal Medicine, 22E, pp. 1911-1912
Goldman-Cecil Medicine, pp. 422-425

The 12 ECG Leads

The standard 12-lead ECG views the heart from 12 different "camera angles":

6 Limb (Frontal Plane) Leads

Lead	Orientation
I	Left arm (+) vs. right arm (-); 0°
II	Left leg (+) vs. right arm (-); +60°
III	Left leg (+) vs. left arm (-); +120°
aVR	Right arm (+); -150°
aVL	Left arm (+); -30°
aVF	Left foot (+); +90°

These are arranged on a hexaxial diagram to calculate the mean QRS electrical axis.

6 Precordial (Horizontal Plane) Leads

V1, V2: right precordium (right ventricle view)
V3, V4: transitional zone (septum)
V5, V6: left precordium (left ventricle view)

A key principle: a depolarization wave moving toward the positive pole of a lead inscribes a positive (upward) deflection; moving away inscribes a negative (downward) deflection; moving perpendicular = biphasic.

Territorial Grouping (for Ischemia/Infarction)

Territory	Leads	Artery
Inferior	II, III, aVF	RCA
Anterior	V1-V4	LAD
Lateral	I, aVL, V5-V6	LCx
Posterior	V1-V2 (reciprocal changes)	RCA/LCx

Harrison's Principles of Internal Medicine, 22E, p. 1912

Systematic Approach to ECG Interpretation

Harrison's recommends evaluating in this order:

Rate - HR (bpm from RR interval)
Rhythm - regular vs. irregular; identify P waves
PR interval - AV conduction
QRS interval - ventricular conduction
QT/QTc intervals - repolarization
Mean QRS axis - normal (-30° to +90°)
P wave morphology - atrial enlargement?
QRS voltages - hypertrophy?
Precordial R-wave progression - V1 to V6 should increase
Abnormal Q waves - old infarction?
ST segments - ischemia/injury?
T waves - ischemia, electrolyte abnormality?
U waves - hypokalemia?

Key Pathological Patterns at a Glance

Finding	Possible Causes
Prolonged PR	First-degree AV block, drugs (digoxin, beta-blockers)
Short PR + delta wave	Wolff-Parkinson-White (WPW)
Wide QRS (>120 ms)	LBBB, RBBB, ventricular rhythm, hyperkalemia, Na+-channel blocker toxicity
ST elevation	STEMI, pericarditis, Brugada, vasospasm (Prinzmetal's)
ST depression	Subendocardial ischemia, NSTEMI, LVH strain pattern
Q waves	Prior MI, cardiomyopathy, normal septal Q in lateral leads
Peaked T waves	Hyperkalemia (early), LVH
Prolonged QTc	Hypokalemia, hypocalcemia, hypomagnesemia, drugs, channelopathies
Osborn (J) wave	Hypothermia
Wellens T-wave pattern	Critical LAD stenosis

Summary

The ECG records cardiac electrical activity through 12 leads, each offering a different spatial perspective on the same events. The sequence - SA node fires → atria depolarize (P wave) → AV node delays → ventricles depolarize (QRS) → ventricles repolarize (T wave) - is visible on every normal beat. By analyzing rate, rhythm, intervals, axis, and morphology of each waveform, the ECG reveals a wide range of diagnoses: arrhythmias, conduction blocks, ischemia, infarction, electrolyte abnormalities, drug effects, and structural heart disease.

Sources: Harrison's Principles of Internal Medicine 22E | Goldman-Cecil Medicine | Miller's Anesthesia 10e | Gray's Anatomy for Students

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